The present invention relates to an interferometer, in particular to an interferometer used for determining a wavelength of light, an absolute distance, a relative displacement and/or a refractive index and/or used for switching optical signals and/or used as a wavelength selective cross-connect and/or used for analysing particle or atomic beams, preferably of electrons, protons, neutrons or atoms, in view of determining e.g. an acceleration and/or rotation thereof and/or used for analysing or determining the surface properties of a substrate such as its flatness.
It is known to analyse light by means of interferometers such as the Michelson interferometer, the Fabry-Perot interferometer and to measure the refractive index by means of interferometers such as the Jasmin interferometer. Moreover, it is known to use diffraction gratings for the wavelength-division multiplexing and demultiplexing. Furthermore, it is known to analyze particle beams e.g. by means of mass spectrometers or the Mach-Zehnder interferometer.
However, the above standard interferometers do not provide information about the absolute distance and the accuracy of the measurement of relative displacements and of the refractive index is limited by the distance between the next fringes of the interferometer, which can not be smaller than one half of the light wavelength. Moreover, the standard wavemeters based on the Michelson interferometer cannot be compact.
Thus, it is an object to provide an improved interferometer, which can be compact and allows for measurements with an improved accuracy.
According to the invention, there is provided an interferometer or switch or refractometer or spectrum analyser or spectrum separator or optical signal multiplexer/demultiplexer or spectrometer comprising:
at least one multimode waveguide or a waveguide having a plurality of modes which can be populated,
(light or matter) beam or wave input means, preferably light input means, for inputting a primary (light or matter) beam or wave, preferably a preferably collimated primary light beam or ray, under a specified (predetermined or predeterminable) primary angle xcex1 into the multimode waveguide so as to generate an interference between two or more populated modes of the multimode waveguide, and
at least one (light or matter) beam or wave processing or analysing or outputting means, preferably at least one light processing or analysing or outputting means, for processing or analysing or outputting one or more secondary (light or matter) beams or waves, preferably light beams or rays, exiting the multimode waveguide.
Accordingly, the interference occurs between two modes (high and/or low modes) of the multimode waveguide having two or more propagated modes. Thus, a preferably monochromatic light or primary light beam and/or matter (particle or atomic) beam (e.g. an electron, proton and/or neutron beam or an atomic beam of preferably ionized atoms) is or can be inputted into the multimode waveguide at an angle xcex1, preferably at some macroscopic angle xcex1. Accordingly, the intensity of the secondary (light or matter) beam or light beam, in particular the intensity of the two secondary matter beams or light beams, going out of or exiting or leaving the waveguide experience a (semi- or quasi-) periodic modulation as a function of the total (optical) pass of the waveguide. The (semi- or quasi-) periodic oscillation of the beam or light intensity in the secondary (matter or light) beam can be explained by the interference between the few populated high modes of the multimode waveguide. In particular, the preferably monochromatic light or monoenergetic matter beam enters the multimode waveguide at some preferably macroscopic angle, when only a few high modes of the waveguide are populated. The beatings between these modes leads to the (semi- or quasi-) periodic oscillations of the intensity particularly in the two (light) beams going out of the waveguide at opposite angles. Due to this specific modulation, the distance between the fringes of such an interferometer can be much smaller than in all standard existing interferometers. Moreover, for preferably large effective length of the waveguide these interference fringes are very sensitive to small variations of the (optical) width and length of the waveguide and also to the wavelength of light or energy of the matter beam. Very small differences between the fringes of the interferometer can be observed, in particular fringes with period of about xcex/9 were already experimentally observed, and allow for an improved resolution of the interferometer in space and frequency of light or energy of the beam.
Thus, the interferometer according to the invention preferably provides a small distance between the transmission fringes which can be used as a fine ruler for the precision positioning or orientation of different objects or the measurement(s) thereof. For example, it can be used for the precise translation of samples in electron lithography and in different types of high-resolution microscopes. The non-uniform structure of the fringes allows to precisely determine the width of the waveguide, in particular the absolute distance between mirrors thereof and/or to precisely determine the refractive index of the waveguide. Moreover, the position of the transmission fringes of the interferometer depends on the light wavelength, which allows to use it for spectrum analyzing or measurement of the wavelength of the light and also for separation and/or mixing of different frequency components of light. Moreover, the inventive interferometer works not only with light but also with preferably monochromatic or monoenergetic particle or atomic beams, e.g. with electron, proton and/or neutron beams e.g. for determining their acceleration and/or rotation with respect to the waveguide.
According to a preferred embodiment of the invention, the primary angle xcex1 satisfies the following condition:
xcex1 greater than arcsin(xcex/d)
wherein d is the width of the waveguide and xcex is the wavelength of the primary beam, preferably light beam. Accordingly, the minimum possible primary angle xcex1 is smaller than the diffractional divergence of the preferably two outgoing (light) beams so that a complete separation of the preferably two outgoing (light) beams is preferably possible.
Preferably, the multimode waveguide comprises two substantially plane mirrors capable of at least partly reflecting the beam, wherein the primary beam, preferably light beam, is (or can be) inputted at a first end face between the mirrors and the secondary beam, preferably light beam, exits the multimode waveguide from between the mirrors at a second end face thereof, wherein the primary angle xcex1 preferably satisfies the following condition:
xcex1xe2x89xa690xc2x0.
Further preferably, the interferometer is used for determining an absolute distance between the mirrors, a relative displacement of one mirror being movable with respect to the other mirror, a relative orientation of the two mirrors and/or a refractive index of the medium positioned between the mirrors, and/or used for analysing a spectrum of the primary light beam,
wherein the light processing means comprises one or more beam detectors, preferably light detectors. Such light detectors preferably may be single photodiodes, an array of photodiodes or a charged coupled device (CCD).
Still further preferably, the interferometer is used for controlling and/or measuring and/or determining one or more surface properties, preferably flatness of an optical substrate, wherein the optical substrate to be controlled is used as one of the mirrors and the other mirror of the mirrors forms a reference substrate. Accordingly, a minimal observable fringe spacing in the multimode waveguide interferometer can be used to determine or measure or control the flatness of the reflecting surfaces of the multimode waveguide interferometer. Thus, it is in particular possible to detect much smaller deviations of surfaces from the plane than other standard methods such as wedge interferometers having an accuracy of less than xcex/10 to xcex/20.
According to a further preferred embodiment, the multimode waveguide comprises a dielectric active material having a refractive index which can be varied preferably by applying a specific (predetermined or predeterminable) voltage to a plurality of electrodes and/or a specific (predetermined or predeterminable) magnetic field and/or an additional light field. Such a dielectric waveguide interferometer made of an active optical material can be preferably used to switch the light between the two different output channels. For example, a waveguide may be made of a lithium niobate crystal (LiNbO3) and/or of other nonlinear crystals. Accordingly, in one preferred embodiment it is possible to control or modify the effective refractive index of the multimode waveguide with additional control light pulses or beams. Thus, it is possible to use the interferometer as a switch controlled by or based on the light-induced switching between the fringes of the non-linear multimode waveguide interferometer, the switching time being estimated to be as fast as 10 ps.
Preferably, the primary beam is a primary light beam and the primary angle xcex1 satisfies the following condition:
xcex1xe2x89xa690xc2x0xe2x88x92arcsin(1/nw)
where nw is the refractive index of the dielectric material, in particular at the specific voltage. For example, for a waveguide made of lithium niobate (LiNbO3) having a refractive index of n=2.15 the maximum angle is preferably 62.3xc2x0. Further preferably, the multimode waveguide comprises a planar dielectric material and is provided on or above a surface of a dielectric substrate. For instance, a lithium niobate LiNbO3 crystal may be built on/above a quartz substrate.
Most preferably, the primary angle xcex1 satisfies the following condition:
xcex1xe2x89xa690xc2x0xe2x88x92arcsin(ns /nw)
where ns and nw are the refractive index of the substrate and of the waveguide, respectively. Accordingly, the maximum angle takes into account the critical angle of total reflection. E.g. for a lithium niobidate waveguide having nw=2.15 on a glass substrate having ns=1.5 the maximum possible angle is about 46xc2x0.
According to a further preferred embodiment of the invention, the multimode waveguide is a one-dimensional multimode waveguide or a waveguide in which the propagation of light is quantized in one dimension while it is not quantized in the other two dimensions. Accordingly, the optical paths can be predicted preferably without the use of numeric calculations or estimations.
Most preferably, the interferometer is used for switching optical signals and/or is used as a wavelength selective cross-connect,
wherein the primary beam or wave is a primary light beam and the secondary beam or wave is a secondary light beam,
wherein the light processing means comprises two light output means such as optical fibers and
wherein a plurality of electrodes is provided on the waveguide to apply a specified (predetermined or predeterminable) voltage.